Why is frazil ice usually the first ice to form in a river?

Frazil ice is typically the first ice to form in rivers because of the unique way moving water behaves compared to the still water found in lakes.

In rivers, turbulence causes the entire depth of the water to cool uniformly. Once the water temperature reaches the freezing point and continues to lose heat, the water enters a state of supercooling. During this process, the introduction of ice particles from the air triggers the nucleation of ice throughout the flow.1

These small, disk-shaped particles are known as frazil ice. Because the water is supercooled, these particles are inherently unstable and actively grow, causing them to adhere to one another or to submerged surfaces upon contact. This formation is the primary ice formation mechanism in rivers, as the constant mixing prevents the formation of a stable, solid surface sheet that might otherwise develop in calmer bodies of water.1